The figurative meaning of this phrase is a bit obscure. However, here's the origin of it: The phrase dates back to 1896 and was the subject of an article in the New York Sun titled “Johnny on the Spot: A New Phrase Which Has Become Popular in New York.” The article details the expression that had become very popular in NYC. According to the writer, the phrase is from a slightly longer version, ‘Johnny is always on the spot when wanted.’ … where Johnny refers to a general male (like John Doe). Although, the author does venture to provide a little more detail on who Johnny is: “a man or youth who may be relied upon to be at a certain stated place when wanted… an individual who is prompt and farseeing, alive to his own interests and keenly sensible of means for promoting his own advantage is a ‘Johnny on the spot.’” (source)
The figurative is derived from the origin but has expanded to mean any person (male or female) who is on hand and ready to perform a service or respond to an emergency.